Various configurations of metal sheath heating elements are known in the prior art. A typical tubular heating element comprises a coiled resistance wire extending coaxially along the length of an elongate metal sheath. An insulating material having an optimum combination of relatively high thermal conductivity and relatively low electrical conductivity is used to fill the space between the coil and the inner wall of the sheath. Granulated magnesium oxide is one substance known to be suitable for the purposes of serving as the insulating filler material. Toward the end of the manufacturing process, the granulated magnesium oxide is introduced into the sheath, for example by gravity feed. Upon sealing the sheath, the sheath is subjected to compression forces, for example, by swaging, pressing, or the like, thereby compacting the granulated magnesium oxide to improve its dielectric and thermal conductive properties.
Depending upon the intended application, metal sheath heating elements of varying sizes and voltage ratings may be required. In many applications, a relatively high voltage, on the order of 480 volts or so, may be desired.
Metal sheath heating elements are often used in radiant heaters for industry, for providing comfort heating in unheated areas such as steel mills, loading docks, and maintenance areas, for example. In such applications, the heating elements are subjected to seasonal usage cycles. During the summer months, the heating elements are not energized. Depending on the humidity, such seasonal usage provides an opportunity for the insulating filler material, which is typically hygroscopic, to absorb moisture, for example, through the terminal ends of the metal sheath from which the electric supply wires extend.
If moisture is permitted to accumulate within a heating element's sheath and the heating element is then energized, the moisture inside the sheath is driven by heat to the colder terminal ends and can accumulate in high concentration. If the accumulation of moisture becomes high enough, the electrical resistivity of the insulating filler material may diminish to the point that the insulating material may not be able to withstand the line voltage, particularly in high-voltage heaters. Depending upon the amount of moisture involved, this can lead to leakage current through the metal sheath. In some cases, the heater may short to ground through the metal sheath. In the worst cases, complete breakdown can occur.
In some applications, a metal sheath heating element can be provided with hermetic terminal seals to completely prevent moisture infiltration into the sheath. One known approach is to provide a seal consisting of a high-grade, dense, cylindrical ceramic insulator fused to a metal component part at each end of the sheath, the ceramic insulator then being brazed to the sheath itself. Other approaches to hermetic sealing of heating elements are known. Hermetic sealing may not be appropriate in all circumstances, however, particularly in high-temperature applications. Even if a heating element is hermetically sealed, a residual amount of oxygen typically remains within the sealed sheath. At very high temperatures, the interior surface of the sheath and the coiled heating wire itself will tend to react with any oxygen present (i.e., oxidize). Once the residual oxygen is depleted through such oxidation, any further oxidation of the sheath and heating wire can only occur through breakdown of the magnesium oxide filler material. As this occurs, the filler material tends to become increasingly "metal rich," tending to adversely impact its dielectric properties. This leads to poorer heating element performance and can ultimately lead to complete breakdown of the element.
The aforementioned problems relating to oxidation of the filler material can similarly occur even if the sheath is not hermetically sealed. This effect is even more pronounced in very long tubular heating elements.